Dielectric block filters are well known art. U.S. Pat. No. 4,431,977, for example, discloses a ceramic block filter. Numerous other U.S. patents disclose improvements that these devices have realized over the past few years.
Ceramic block filters have found wide acceptance for use in radio communications devices, particularly high frequency devices such as selective call receivers (pagers), cellular telephones, and other two-way radio devices. The blocks are relatively easy to manufacture, rugged, have improved performance characteristics over discrete lumped circuit elements, and are relatively compact.
For purposes largely related to simplified manufacturing prior art ceramic block filters are designed and constructed to have substantially identical input and output impedances; input and output ports of a ceramic block filter are frequently constructed so that the filter blocks have virtually identical or symmetrical input and output impedance values. As a consequence of ceramic block filters being designed and constructed to have symmetrical input and output impedance values, their use in a radio communications device frequently necessitates the addition of impedance matching networks to accomplish maximum power transfer through them.
Consider for example the antenna's frequently used in cellular telephones, which might have input impedances of approximately 50 ohms. In contrast, the output power amplifier stage of the transmitter section of a cellular phone can have an output impedance that is substantially lower, frequently less than 20 ohms. Most ceramic block filters have a characteristic 50 ohm input and a 50 ohm output impedance. To accomplish maximum power transfer between the power amplifier stage and the antenna, an impedance matching network must be inserted between the output of the power amplifier stage and the input of the transmit filter, when the transmit filter output is coupled directly to the antenna.
FIG. 1 discloses a block diagram of a portion of a radio communications device (10-1) known in the prior art. The power amplifier stage (12), with an output impedance value of Z.sub.1 requires an impedance matching network (14) to maximize the power transfer from the power amplifier stage (12) to the antenna (18) through the ceramic block transmit filter (16). The impedance matching network (14) has an input impedance Z.sub.2 that is substantially identical to the output impedance Z.sub.1 (or the complex conjugate) of the power amplifier (12). Similarly, the impedance matching network (14) has an output impedance Z.sub.3 that is substantially identical to the input impedance (or the complex conjugate) of the transmit filter (16), Z.sub.4. As is well known in the art, the transmit filter (16) output impedance Z.sub.5 is preferably equal to or near equal to the input impedance Z.sub.6 of the antenna (18).
FIG. 3 discloses a simplified block diagram of a portion of a radio receiver apparatus 11-1. In FIG. 3 the antenna (18) has a characteristic impedance substantially equal to the input impedance Z.sub.10 (or the complex conjugate) of the receiver filter stage (100). The receiver filter (100) which is a ceramic block filter has an output impedance Z.sub.11 that is substantially equal to the input impedance Z.sub.12 (or the complex conjugate) of an impedance matching network (22). The impedance matching network (22) is constructed to have an output impedance Z.sub.13 substantially equal to the input impedance Z.sub.14, or the complex conjugate, of the rest of the amplifier stage represented by circuit block (24) and labelled as an amplifier/mixer.
A ceramic block filter that could eliminate the need for an impedance matching network between signal processing stages in a radio communications device would be an improvement over the prior art.